The most frequently used methods today of producing clearcoats for automotive and other coatings applications involve application of the clearcoat composition over a high-solids basecoat. The basecoat is usually composed of one or more hydroxy-functional polymers and an aminoplast crosslinking agent. Curing of the high-solids basecoat generally requires the presence of a strong acid catalyst (e.g., p-toluenesulfonic acid, dinonylnaphthalenedisulfonic acid, dodecylbenzenesulfonic acid, phenyl acidphosphate).
Clearcoat compositions can be characterized as being of the one-component type or the two-component type. In the two-component or "two-pack" system, the polymer and the cross-linking agent are combined just prior to coating. The two-component system allows for the use of highly reactive cross-linking agents, which can provide advantageous physical properties to the coating. However, the process usually requires mixing of the components in the right proportion immediately prior to coating, and can be expensive to operate and difficult to control. The one-component or "one-pack" systems offer significant advantages in manufacturability because the cross-linkable clearcoat coating composition can be coated as one formulation. However, when highly reactive crosslinking agents such as polyisocyanates are used, the cross-linking agent in one-component systems must be blocked in order to prevent premature cross-linking of the clearcoat composition. The blocking group can then be unblocked under specified conditions, such as high heat, to allow the materials to crosslink so the coating can be cured. The most resins (e.g., melamine formaldehyde resin) and isocyanates (e.g., oxime-blocked isophorone diisocyanate).
Clearcoats that have been cross-linked with aminoplast cross-linking agents exhibit good clarity and hardness; however, they suffer from a phenomenon known as environmental etch. Environmental etch appears as milky or cloudy marks on clearcoat finishes that have been exposed to the elements. Clearcoats that have been cross-linked with blocked isocyanate cross-linking agents also exhibit good hardness, and they are resistant to environmental etch. However, they suffer from severe yellowing during heat curing Unlike coatings derived from unblocked isocyanates, where yellowing is primarily limited to aromatic polyisocyanates, coatings derived from blocked isocyanates exhibit significant yellowing during cure even when an aliphatic polyisocyanate is used.
The use of hydrazide compounds having structural units according to the formula: EQU --CO--NH--NH--CO--
reduce yellowing in two-component polyurethane coatings utilizing unblocked polyisocyanates has been described in U.S. Pat. No. 4,369,301. These compounds have also been used in one-pack acrylic coatings utilizing blocked polyisocyanate curing agents, as described in U.S. Pat. No. 5,112,931. This patent alleges that the use of such structures provides the coating with a flat finish. One compound containing the above structure is phthalhydrazide, which has the structure: ##STR1## However, when phthalhydrazide is used in one-component systems with blocked polyisocyanates, it has little effect on reducing yellowing.
It is thus an object of this invention to provide a colorless or clear coating utilizing a one-component clearcoat that has the environmental etch resistance provided by blocked isocyanate cross-linking agents, but with reduced yellowing.